Ion trap mass spectrometers, or quadrupole ion stores, have been known for many years and described by several authors. They are devices in which ions are formed and contained within a physical structure by means of electrostatic fields such as r.f., DC and a combination thereof. In general, a quadrupole electric field provides an ion storage region by the use of a hyperbolic electrode structure or a spherical electrode structure which provides an equivalent quadrupole trapping field.
The storage of ions in an ion trap is achieved by operating trap electrodes with values of r.f. voltage V and associated frequency f, DC voltage U, and device size r.sub.0 and z.sub.0 such that ions having mass-to-charge ratios within a finite range are stably trapped inside the device. The aforementioned parameters are sometimes referred to as trapping parameters and from these one can determine the range of mass-to-charge ratios that will permit stable trajectories and the trapping of ions. For stably trapped ions the component of ion motion along the axis of the trap may be described as an oscillation containing innumerable frequency components, the first component (or secular frequency) being the most important and of the lowest frequency, and each higher frequency component contributing less than its predecessor. For a given set of trapping parameters, trapped ions of a particular mass-to-charge ratio will oscillate with a distinct secular frequency that can be determined from the trapping parameters by calculation. In an early method for the detection of trapped ions, these secular frequencies were determined by a frequency-tuned circuit which coupled to the oscillating motion of the ions within the trap and allowed the determination of the mass-to-charge ratio of the trapped ions (from the known relationship between the trapping parameters, the frequency, and the m/z) and also the relative ion abundances (from the intensity of the signal).
Although quadrupole ion traps were first used as mass spectrometers over thirty years ago, the devices had not gained wide use until recently because the early methods of mass analysis were insufficient, difficult to implement, and yielded poor mass resolution and limited mass range. A new method of ion trap operation, the "mass-selective instability mode" (described in U.S. Pat. No. 4,540,884), provided the first practical method of mass analysis with an ion trap and resulted in the wide acceptance and general use of ion trap mass spectrometers for routine chemical analysis. In this method of operation, which was used in the first commercially-available ion trap mass spectrometers, a mass spectrum is recorded by scanning the r.f. voltage applied to the ring electrode whereby ions of successively increasing m/z are caused to adopt unstable trajectories and to exit the ion trap where they are detected by an externally mounted detector. The presence of a light buffer gas such as helium at a pressure of approximately 1.times.10.sup.3 Torr was also shown to enhance sensitivity and resolution in this mode of operation.
Although the mass-selective instability mode of operation was very successful, a newer method of operation, the "mass-selective instability mode with resonance ejection" (described in U.S. Pat. No. 4,736,101) proved to have certain advantages such as the ability to record mass spectra containing a greater range in abundances of the trapped ions. In this method of operation, a supplementary field is applied across the end cap electrodes and the magnitude of the r.f. field is scanned to bring ions of successively increasing m/z into resonance with the supplementary field whereby they are ejected and detected to provide a mass spectrum. Commercially-produced ion trap mass spectrometers based on this mode of operation have recently become available, and these instruments have been successfully applied to an even wider variety of problems in chemical analysis than their predecessors.
The capabilities of quadrupole ion traps have continued to expand since the development of the mass-selective instability modes of operation described above. The versatility of these relatively simple mass spectrometers has been demonstrated by their high sensitivity in both electron and chemical ionization and their ability to serve as gas-phase ion/molecule reactors. The successful introduction of externally produced ions into these devices has even allowed the study of biomolecules using such techniques as laser desorption, cesium ion description, and electrospray ionization. The ion storage ability of the quadrupole ion trap makes possible tandem mass spectrometry (MS/MS) (U.S. Pat. No. 4,736,101) involving many stages of mass analysis with efficient dissociation of ions. Even parent MS/MS scans have been reported. The usable mass range of these mass spectrometers has been extended to 45,000 daltons (for singly charged ions) and beyond.
Despite these capabilities, a limitation of the ion trap mass spectrometer as compared to other types of instruments, such as sector (including three- and four-sector) instruments or Fourier transform-ion cyclotron resonance instruments, is the constraint of always operating at a relatively low resolution.